Garnet in the Neoproterozoic Oddanchatram Anorthosite in the Southern Granulite Terrain, India: A Petrological Puzzle

Garnet is a commonly occurring accessory mineral in many anorthosite plutons, and it is variably interpreted as a metamorphic, magmatic, or xenocrystic mineral. The Neoproterozoic Oddanchatram anorthosite in the South Indian granulite terrain is one such example. Igneous textures are remarkably well preserved, with large grains (up to 5 cm) of garnet in the margins of the pluton. Typical prograde metamorphic textures, such as garnet formation by isobaric reaction between mafic silicate and plagioclase or by dehydration breakdown of amphibole, are lacking, and Fe-Ti oxides mostly occur as exsolutions in the associated plagioclase or scarce disseminated oxide grains. The garnets are almandine rich in composition, with higher CaO (>5 wt%) than the low-CaO (